Yipe. I also write fiction as a hobby and I purposefully don't want criticism of it, that's why I only let my parents read it, with the proviso that they can't say anything critical (which I think is something you can only ask of your parents!). My feeling is that if I put something out there for other people to read, I have to accept that they may not like it, or may criticize various points (hopefully, in a constructive and not mean way).
Of course not everyone realizes that. At least Jason's stories are short, and you don't have to slog through a long, horrible novel. I like the idea of redirecting him towards other writers' resources, like classes, groups, and forums.
If you want, you could sound him out on constructive criticism by offering up a smidgen of it to see how he reacts--maybe something like what mime said, "Your nonfiction and historical pieces are so charming and engaging that I'm honestly disappointed to see you moving away from that style." I think that's a very gentle, but definite, criticism. If he seems keen to know more, you can continue doling out some constructive remarks. If he seems a bit affronted, you can pull back.